Pickling of magnesium and magnesium-base alloy articles



United States 39,100,169 Patented Aug. 6, 1963 No Drawing. Filed Jan. 4, 1960, Ser. No. 82 3 Claims. (Cl. 156-20) The invention relates to pickling magnesium metal and magnesium-base alloys containing at least about 75 percent by weight magnesium to improve the corrosion resistance thereof. Hereinafter articles of either magnesium metal or magnesium-base alloys of such composition will be sometimes referred to merely as magnesium articles The invention more especially relates to pickling articles of magnesium-base alloys particularly those containing thorium or rare earth metals, e.g., misch metal, to remove corrosion-stimulating surface contaminants to improve the corrosion resistance thereof and impart a bright, shiny, lustrous finish on the surface.

Magnesium articles, which contain surface contamination in the form of heavy metals, e.g., iron and copper, are extensively corroded when contacted by aqueous solutions or condensation from the ambient atmosphere, there-by weakening, hastening deterioration and strength failure, and detracting from the appearance thereof.

A number of attempts have been made to remove contaminants from the surface .of magnesium articles to brighten the surface and lessen corrosion thereof. Among such attempts have been chemical treatment, usually consisting of dipping the magnesium article in an aqueous solution of a chemical composition, or otherwise applying such aqueous solution to the magnesium article followed by Water-rinsing and usually by drying. Among such chemical compositions are dilute aqueous solutions of certain acids and aqueous solutions of glycolic acid and a soluble nitrate.

Known cleaning or pickling compositions for the removal of such surface contaminants thereof have not been fully satisfactory. Known cleaning compositions have one or more of the following disadvantages associated with their use: they remove too much metal or remove it too fast for proper control; do not adequately protect the magnesium metal against corrosion; are offensive or otherwise impractical to use; are relatively expensive; are responsible for large losses .of cleaning composition due to drag-out when the magnesium article being cleaned is removed from the cleaning composition; cannot be revivified and therefore must be frequently replaced; or are not applicable to cleaning a number of magnesiumbase alloys.

A need therefore exists for an improved cleaning composition and use thereof for cleaning magnesium articles and magnesium-base alloys to improve the resistance thereof to corrosion. A particular need exists for such a composition and use thereof for cleaning articles of a magnesium-base alloy containing misch metal or other rare earth metals and for magnesium-base alloys containing thorium, and particularly for a cleaning composition and use thereof whch also imparts to such magnesiumrare earth and magnesium-thorium alloy articles a bright lustrous finish.

The invention meets this need by providing a method of cleaning magnesium-base alloy articles by employing a cleaning composition which require but a short treating time, provides good protection against corrosion with relatively small loss of metal during treatment, leaves a smooth surface on the articles, can be used for protracted periods of time, is exceedingly readily obtained at low cos-t, and can be revivified as found necessary.

The invention is a method of removing contaminants from the surface and improving the resistance of a magnesium metal and magnesium base alloy article to corrosion which consists of immersing the article or otherwise applying to the article an aqueous solution of between 20 and 35 normal H for a time sufficient to remove substantially all foreign matter, tarnish, discoloration, and scale from the surface of the article, removing the article from the solution, and water-rinsing the article thus treated. It further provides a bright metallic finish on magnesium alloys containing less than 0.5 percent Zn.

The method of the invention is particularly effective to provide a smooth bright finish on magnesiumabase alloy of magnesium and thorium or of magnesium and rare earths which may also contain other common alloying metals, e.g. Mn, Ca, Si, Cu, Fe, Ni, Bi, Ag, Li, Pb, Zr, Al, and Zn. The Zn content is preferably not over 0.5 percent. Although the cleaning composition employed in the invention is highly selective for removing contaminants from the surface thereof which normally cause poor resistance to corrosion, some magnesium basis metal is removed in the course of treatment. The depth of metal removed from the articles being treated varies according to the type of magnesiumbase alloy being treated and the character of the tarnish or scale on the surface of the article. Extruded magnesium-base alloy articles and articles formed from a magnesium-base alloy which has previously been pickled to remove contaminants from the surface thereof may be adequately protected against corrosion by the removal of not over 0.1 mil of metal per side during treatment according to the invention. Treatment of thorium-zirconium-magnesiumbase alloys is usually successful when a total of not over 0.1 mil of metal is removed. For common rolled stock, 0.3 mil of metal is usually removed and sometimes as much as 0.5 mil is necessary to insure adequate corrosion resistance.

If the normality of the aqueous sulfuric acid employed is less than about 20 normal, the rate of dissolution of metal from the magnesium article being pickled is too fast for proper control. Furthermore, when the normality is below 20 normal, not only is an excessive amount of metal removed from the surface of the article, but improved resistance of the article to corrosion is unsatisfactory.

The contaminating iilm or scale (formed on a magnesium article due to the corrosive effect of ambient fluids, e.g., air encountered by the article during shaping, fabrication, and storage, is not to be confused with the dark stain that is formed during the treatment according to the invention on magnesium alloy articles containing more than 0.5 percent zinc. Good resistance to corrosion is attained by treatment of such alloys in aqueous H 50 of a normality between 20 and 35 in accordance with the invention despite the resulting dark stain. Therefore, where such dark stain formed during pickling is not objectionable, magnesium-zinc alloys may be treated in such acidic solution and such treatment falls within the purview of the invention.

When sulfuric acid having a normality of above 35 normal is employed, the pickling process is too slow for practical application. However, completely concentrated sulfuric acid may be employed if the time required is not objectionable. Lower concentrations of the sulfuric acid within the range employed in the invention, i.e., those approaching 20 N, decrease the time necessary to provide the resistance to corrosion attainable by the practice of the invention whereas the higher concentrations of sulfuric acid, i.e., those approaching 35 N permit :a longer period of treating and do not require as close supervision of the 3 treating or pickling operations as when a more dilute acid is used.

The temperature of the pickling bath employed is not highly critical. The lower the temperature of the bath, the slower is the cleaning and brightening process, conversely, the higher the temperature of the bath, the faster the cleaning and brightening process. It the customary therefore, to employ a lower temperature with the less concentrated acid, e.g., 20 to 25 normal, and a higher temperature with the more concentrated acid, e.g., 30 to 35 normal. The preferred normality of the sulfuric acid to employ is between 20 and 30 normal. The preferred temperature to employ is between about room temperature, say 80 F., and about 150 F., however, temperatures up to 400 F. are satisfactory. When employing an acid within the recommended ranges of normality and temperature, i.e., from 20 to 25 N and from :80 to 150 F., a treating period of between 0.1 and 7 minutes is usually employed, depending upon the concentration and temperature. Ah'eating period, however, of up to 3 hours could *be employed, if desired, employing a relatively low temperature and high concentration of acid solution for removal of an especially bad scale.

The rate of metal removal does not effect the results of the invention. Employing a sulfuric acid concentration of high normality and/ or low temperature will attain a desirable result, although requiring a longer reaction time, than employing a low normality acid of about 20 N and/ or a high temperature.

It will be noted that the process of the invention produces a satisfactory corrosion-resistant surf-ace to many magnesium-base alloy articles by removing not more than 0.1 mil of metal and, for those alloys which have deep -or otherwise more difficulty removable contaminants thereon,. by removing only 0.3 mil of metal which is definitely lower than that now removed by known methods employing conventionalpickles. The sulfuric acid, having the required normality range of the invention, attacks the contaminants on the surface of the magnesium article in preference to the magnesium metal.

Known pickling compositions require the presence of a sequestering agent to prevent objectionable ions present in the composition from interfering with the pickling process. The method of the invention, employing sulfuric held within the range set forth, tolerates relatively high concentrations of heavy metal impurities, known to decrease corrosion resistance of known pickles extensively. The sulfuric acid in the range employed in the invention serves as its own sequestering agent. For example, a panel of magnesium-base alloy consisting chiefly of magnesium and thorium, of 0.064 inch gauge, was pickled for a sufficient time to remove one-half mil per side in a 22 normal aqueous solution of sulfuric acid in the presence of 10,000 parts per million of iron in the pickle. After removal from the pickle and water-rinsing, it had a bright lustrous metallic surface, entirely free from any deposition on the surface thereof (despite the presence of iron in the bath), and showed a corrosion rate of only 0.23 milligram per em. per day as determined in a standard corrosive salt solution test, (as described hereinafter). Another panel of the magnmium base-thorium alloy was pickled in an iron-free bath of 22 normal sulfuric acidand then subjected to the same salt solution test. The corrosion rate of the thus pickled panel was again 0.23 milligram per om. per day.

The invention is practiced by subjecting a magnesiumarticle to be cleaned and brightened to the action of a 20 to 35 N aqueous solution of H 80 at a temperature and for a time sufficient to remove substantially all the foreign matter, tarnish, scale, and the like from the surface of the article, usually at least 0.1 mil per side and, preferably at least 0.3 mil of metal per side from the article, removing the thus-treated article from the H 80 solution, and rinsing it with water. It is preferably dried after rinsing. The invent-ion is usually practiced by immersing 4 the magnesium articleto be treated in a bath of the aqueous H of suitable normality, although the aqueous H 80 may be sprayed thereon or otherwise applied.

The metal to be given corrosion protection according to the invention is preferably first dipped into, or has sprayed thereon, a conventional degreasing solution, which usually consists of an aqueous solution of NaOH and/ or Na CO e.-g., one made by dissolving 2 ounces of NaOH and 3 ounces of Na CO -l0l-I O per gallon of water.

Best appearance of the magnesium article treated according to the invention is obtained when the article is Water-rinsed promptly, e.g., within 15 seconds and preferably within about 3 seconds after it has been removed from the aqueous H 50 solution to stop, more-or-less abruptly, the action of the acid on the metal after removal from the bath.

The following tests include examples which are illustrative of the practice of the invention:

A number of 1 /2 X 2" X .064" panels of a magnesium alloy composed, in percent by weight, of 0.7 Zr, 3.0 Th, and balance Mg, were prepared. A series of the panels, thus untreated was subjected to a corrosive salt solution according to the procedure set out in the test designated B192-44T in A.S.T.M. Standards 1958, page 243. The

test consisted essentially of weighing a test specimen and alternately (1) immersing the specimen in a 3 percent NaCl brine for 0.5 minute and (2) exposing it to air for 2.0 minutes at a temperature of 80 F. over a period of 7 days, cleaning the corroded material from the surface as by immersion in an aqueous chromic acid solution followed by water-rinsing, drying, gauging for thickness, and reweighing the test specimens. The untreated panels showed an average corrosion rate ranging from 62 to 67 milligrams per square centimeter per day.

Each of the remaining panels was treated by immersion in a different normality aqueous H 50 solution at the temperature and for the time set out in Table I below. After the period of immersion, each of the panels was removed therefrom, water-rinsed, and dried, and the surface thereof examined for brightness. Thereafter, each panel was gauged, weighed and subjected to the ASTM standard salt solution test employed for the untreated panels.

The corrosion rate as shown by the salt solution test is set forth in milligrams per square centimeter per day in Table I. Tests 1, 2, and 3 are blanks for comparison. Tests 4, 5, 6, and 7 are examples of the practice of the invention.

Table I Mils re- Appear- Temp. Pickling moved Corrosion once of bath, t me, by pickle rate, after F. minutes bath MOD 1 pickling 80 0. (14 D. 5 16 -20 Bright.

1 Weight loss in milligrams per om. per day by ASTM test 13192-4 21.

Test runs 4, 5, 6, and 7 of Table I, performed according to the invention, show pronounced improvement in corrosion resistance. The results of the test runs in the table show that the higher concentrations of acid required increasingly long pickling times to attain comparable improvement in corrosion resistance even at advanced temperatures. The corrosion resistance, however, as shown by the salt solution test, remains good when the more concentrated acid was employed. The results of the test runs in the table show that a bright finish 'was obtained as well as excellent resistance to corrosion in aqueous H 80 having on normality range of between 20 and 35 normal H 50 at a temperature between 80 and 300 F. when 0.5 mil of metal is removed from each side. The results also indicated that an aqueous solution of between 20 to 30 N and preferably between 20 and 25 N give best results and require a shorter treating period.

solution and the other a conventional pickle consisting of an aqueous solution of 25 percent glycolic acid and a suflicient amount of magnesium nitrate to yield about 1 unit weight of (N to unit Weights of the glycolic 5 acid, i.e., 1 mole of Mg(NO to about 8 moles of gly To show that the practice of the invention is applicable colic acid. The temperature of the hath-s was about to a large number of magnesium-base alloys, 1 /2" x 2" 80 F. Two 1 /2" x 2" panels, one selected from each of magnesium panels of various such alloys, were cut from the two alloys set out in Table IV, Were treated in the various gauge magnesium alloy sheets having the comsulfuric acid solution according to the invention and two position and thickness set forth in Table I I and pickled 10 more panels, one from each of the same alloys, were for a sufiicient time in a 23 N H 50 bath to remove the treated in the conventional glycolic acid-magnesium nicontaminants from the surface thereof. The depth of trate pickle for a time sufficient to remove the depth of metal removed per side and the improvement in corrosion metal in mils set forth in Table IV. Where the depth resistance as shown by the ASTM salt solution test em of removal is marked 0*, the metal was not treated. The ployed above were also determined and are set out in the V purpose of showing the corrosion nate before pickling table. was to show the effectiveness of the pickle in improving Table II Metal Corrosion rate of metal in MOD 1 Thickness removed Mg-basealloycontaininginpercentbyweight ininches per side,

of panel mils Unpickled Pickled Appearance alter pickling 0.120 0.35 11. 4-1a2 0. Bright. 0.060 0.30 15 -17 0.23 Do. 0.060 0.35 0.30 Brown film. 0. 040 0. 40 12. 943.2 0.29 Black film. 0. 040 0. 40 a. 1- 4. 6 0. 33 Do. 0. 030 0. 1.1 0. 33 D0. 0.100 0. 1.42 0.36 Bright.

1 Weight loss in milligrams 2 These alloys (as are shown) contain more than 0.5 percent zinc.

An examination of the results of Table II, employing 23 normal H 80 at 80? F., showed a pronounced reduction in the corrosion rate as determined by the salt solution test when at least 0.3 mil per side of metal was removed during the pickling treatment. A bright lustrous finish was obtained by the pickling process in all metals containing not more than 0.5 percent zinc. The surfaces of all the magnesium alloy panels treated were smooth and exhibited no pitting or roughened areas after the pickling treatment.

To show the relationship between the depth of metal removed and length of time of immersion in various normality concentrations of H 80 at 140 F., 1 /2" x 2" panels of 0.60 thickness magnesium-base alloy, composed of 0.6 percent Mn, 20 percent Th, and balance magnesium, were each immersed for 2 minutes in a sulfuric acid solution having the normality set out in Table III below, rinsed, and dried, and the thickness of the layer of metal removed from each side determined by gauging. Table III H 50 normality Mils of metal reof pickle: moved per side An examination of the results of Table III shows that than an increase in normality of the sulfuric acid lessens the amount of metal removed during a given period of time. It is apparent from an examination of Table III that where 0.3 to 0.5 mil of metal per side is to be removed at 140 F., two minutes are too long a period of time to employ when using a 20 to 23 normal solution of H 80 that two minutes are quite satisfactory for a 25 normal H 80 but that two minutes is insufficient time when employing a 28 or 33 normal H 80 To show the eifectiveness of removing contaminants from the surface of magnesium articles according to the practice of the invention, in contnast to a method employed to remove such contaminants according to a widely practiced conventional method, two pickling baths were prepared, one consisting of a 23 normal H 80 aqueous per cm. per day according to ASTM test Bl92-44'I.

resistance to corrosion. The corrosion resistance was determined according ot the salt solution corrosion test employed in the tests set out above.

1 Weight loss in milligrams per cm per day.

2 Aqueous solution of glycolic acid and Mg(NOa)2.

3 The metal was not pickled and serves as a blank for comparative purposes.

An examination of Table IV shows that the widely used conventional pickle was definitely inferior in its eihcacy to remove contaminants from the snrtace of the magnesium alloy articles and thereby lessen corrosion when compared to the sulfuric acid solution employed according to the invention.

A number of advantages are to be realized from the practice of the invention, among which are:

(l) Imparting to the magnesium metal articles treated according to the invention a marked and unexpected improvement in corrosion (resistance; (2) removal of less metal than heretofore found necessary in attempts to improve corrosion resistance; (3) the employment of an inexpensive bath readily available which may be revivified as often as desired by merely decanting, filtering, centriringing, or otherwise removing the supernatant sulfuric acid solution from the magnesium sulfate and other salts formed which settle to the bottom thereof during use, and adding on occasion, enough H of sufiiciently high concentration to the thus removed acid to bring the normality thereof up to at least 20 normal; (4) the employment of a pickling bath which maintains its eflicacy in the presence of relatively high percents of contaminating heavy metal ions such as those of iron and copper; a method which produces a bright lustrous shiny finish on magnesium metal and magnesium metal alloys containing not more than 0.5 percent zinc, and especially alloys consisting essentially of magnesium and thorium or of magnesium and the rare earths or mixtures of rare earths such as those designated rnisch metal; (6) a method employing a composition which is readily washed away with water and has no detrimental effect of any type on the metal being treated.

The method of the present invention employing sulf-uric acid of a concentration set forth is not to be confused with known efforts to use concentrations of low normality aqueous H 80 as the pickling solution. Such pickles do not satisfiactori-ly remove contaminants nor provide adequate resistance to corrosion regardless of the depth of metal removed from the surface by treatment with such dilute pickle solutions. The practice of the invention is especially applicable to magnesiumbase alloys containing thorium and/or rare earths and not over 0.5 percent zinc. However, the corrosion resistance provided for other magnesium-base alloys including those containing greater than 0.5 percent zinc is adequate and the invention is applicable thereto where a bright metallic luster on the article produced is not essential. The duration of treatment according to the invention is dependent upon the time necessary to remove substantially all contaminants on the surface of the article being treated. However, although the H SO bath of the required normality is selective in attacking the contaminants in preference to the basis metal, up to 0.1 mil of basis metal is removed when treating a number of alloys and in instances of especially stubborn contaminating scale, 0.3 and in some instances, 0.5 mil of metal is removed.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

l. The continuous method of treating an article composed by weight of at least 75 magnesium, and containing less than 0.5% by weight zinc, to improve the corrosion resistance thereof and impart thereto a bright lustrous finish, which consists of immersing the article in an aqueous bath of H having a normality of between 20 and 30 for a period of time sufficient to remove at least 0.1 mil of metal per side, removing the article thus treated from the bath, water rinsing the removed article, and revivifying said bath by removing the supernatant aqueous solution, leaving a precipitate formed during immersion of the article in the solution of H 80 and adding to the aqueous solution thus remove-d enough H 50 of sufiiciently high concentration to naise the normality of said aqueous solution to at least 20 N, and continuing to immerse articles having said composition in the thus revivified bath.

2. The method of treating a metal article composed by weight of at least magnesium, and less than 0.5% by weight zinc, to improve the corrosion resistance thereof and impart a bright lustrous finish thereto which consists of immersing the article in an aqueous bath of H having a normality of between 20 and 35 N at a temperature between about room temperature and 220 F. for a period of time between about 0.1 and 20 minutes, removing the article from the solution, and water-rinsing the article thus treated.

3. The method according to claim 2 wherein the metal article treated is an alloy comprising at least 75% magnesium, not over 0.5% zinc, and alloying metals selected from the class consisting of thorium and rare earth metals and wherein the time of'treatment is not longer than that required to remove 0.1 mil of metal from each side of the article thus treated.

References Cited in the file of this patent UNITED STATES PATENTS 1,918,545 Hoy July 18, 1933 2,287,050 Miller June 23, 1942 2,751,342 Guggenberger June 19, 1956 2,864,730 Kinder et al Dec. 16, 1958 2,886,420 Jones et a1. May 12, 1959 2,894,883 Walker et a1 July 14, 1959 2,908,557 Black Oct. 13, 1959 2,967,136 Cybriwsky et a1. Jan. 3, 1961 

2. THE METHOD OF TREATING A METAL ARTICLE COMPOSED BY WEIGHT OF AT LAST 75% MAGNESIUM, AND LESS THAN 0.5% BY WEIGHT ZINC, TO IMPROVE THE CORRESION RESISTANCE THEREOF AND IMPART A BRIGHT LUSTROUS FINISH THERETO WHICH CONSISTS OF IMMERSING THE ARTL.ICLE IN AN AQUEOUS BATH OF H2SO4 HAVING A NORMALITY OF BETWEEN 20 AND 35 N AT A TEMPERATURE BETWEEN ABOUT ROOM TEMPERATURE AND 220*F. FOR A PERIOD OF TIME BETWEEN ABOUT 0.1 AND 20 MINUTES, REMOVING THE ARTICLE FROM THE SOLUTION, AND WATER-RISING THE ARTICLE THUS TREATED. 